Just a couple of things that you need to know in order for it to work. Fist make sure you have selected you Graphics card at the bottom of Processing tab in the Mental Ray setup. As you can probably imaging NVidia Mental Ray is best supported by a NVidia card. This is because it uses NVidia’s CUDA technology. Best results are with the new P series cards. Although I have seen speed increases with the K series that’s in my laptop.

Next, we need to change are global Illumination settings. Final Gather is out and not supported for Hardware acceleration and GI Next is in.

The settings are pretty much the same but GI Next will use your Hardware. Giving much faster results. And remember the more graphics cards you have the more hardware acceleration you’ll get.